This invention relates generally to thermosetting coating compositions that are useful for coating substrates such as metal surfaces.
In the manufacture of metal containers, a thin protective synthetic resin coating is applied to the interior of the container. The synthetic resins employed for coating the interior of the metal container are nominally heat-curable polymeric compositions which are applied in the form of a solution or dispersion in a volatile organic solvent. During the drying and baking cycle of a coating operation, there is the problem of contending with the hazard of solvent vaporization and recovery.
The can manufacturing industry utilizes cans which are fabricated from aluminum or steel. The interior of the cans are coated with a thin thermoset film to prevent the contact of the interior metal surface of the can with its contents. Such coatings ideally should have good adhesion to the interior metal surface, low extractables to prevent contamination of the container contents, and a rapid cure rate for economy of container manufacture. Typical synthetic resin coating compositions include vinyls, butadienes, epoxies, alkyl/aminoplasts, and oleoresinous materials. Many of these resinous coating systems have the disadvantage that they require batch premixing just prior to a coating operation, or they require continuous in line mixing in a container coating assembly.
Various technical investigations have addressed the most serious of the problems relating to industrial scale application of protective coatings to articles of manufacture.
U.S. Pat. No. 3,118,848 describes coating compositions which are prepared by mixing together a water-soluble salt of a vinyl polymer, and a water-soluble epoxy or polyhydroxy compound. One or more water-soluble phenol-aldehyde or amino resins, notably water-soluble urea-aldehyde or melamine-aldehyde resins, may optionally be included as curing agents where low baking temperatures are contemplated.
U.S. Pat. No. 3,156,740 describes thermosetting acrylic resin compositions adapted for application as coatings to protect metal surfaces from the action of water, soap, grease, light and prolonged heat. Illustrative of the preparation of a thermosetting resin composition, there are co-reacted (a) a copolymer of 2-10% of acrylic acid, 4.5-88% styrene and 9-93% of 2-ethylhexyl acrylate and (b) 1-epoxyethyl-3,4-epoxycyclohexane, and then there is mixed therein (c) a melamineformaldehyde resin in an amount of 5-50% by weight based on the total non-volatile content of the composition.
U.S. Pat. No. 3,215,756 describes heat-curable mixtures of a vinyl polymer with an epoxy compound in the presence of an amino resin. For example, a methacrylic acid/methyl acrylate copolymer is admixed with a polyglycidyl ether of Bisphenol A and a urea-formaldehyde resin in an organic solvent, and then coated on a substrate and baked to a thermoset film.
U.S. Pat. No. 3,403,088 describes water-dispersed coating compositions which can be applied by electrodeposition. The coating compositions contain an at least partially neutralized acrylic interpolymer and an amine-aldehyde condensation product or a polyepoxide or both.
U.S. Pat. No. 3,418,392 describes a crosslinking composition for an interpolymer (e.g., styrene/n-butyl acrylate/methacrylamide) which consists of a mixture of a polycycloaliphatic polyepoxide (e.g., 3,4-epoxy-6-methylcyclohexylmethyl 2,4-epoxy-6-methylcyclohexanecarboxylate) and a reactive triazine compound (e.g., hexamethoxymethylmelamine). The coating composition is recommended for use in textile print pastes, padding liquor for pigment dyeing of textiles, nonwoven textile impregnation dispersions, and generally as solvent based protective coatings for metal surfaces and the like.
U.S. Pat. No. 3,467,730 describes heat-convertible coating compositions which are prepared from carboxy-containing copolymers, epoxide resins and aminoplast resins. In an example, 37 grams of a 50% copolymer (72% styrene, 20% methyl acrylate and 8% acrylic acid) solution, 6.9 grams of a polyglycidyl ether of Bisphenol A and 8.3 grams of a butylated urea-formaldehyde resin were blended, drawn down on glass, and cured at 200.degree. C. for 30 minutes.
U.S. Pat. No. 3,908,049, describes a method for coating metal surfaces which involves preparing an aqueous dispersion containing a mixture of a neutralized water-dispersible carboxylic acid containing polymer, a water-dispersible heat-curable thermosetting aminoplast or polyepoxide resin and a water-insoluble, long chain monohydroxy alcohol having 8-36 carbon atoms, applying the aqueous dispersion to a metal surface, and baking the coating at 350.degree.-450.degree. F. to volatilize the alcohol and cure the coating.
U.S. Pat. No. 3,960,979 describes a fast curing high solids coating composition which can be applied to the interior of food and beverage cans with a hot melt spray gun. The coating composition is a blend of (a) a low molecular weight epoxy resin (b) a liquid nitrogen resin or phenolic crosslinking agent, (c) a flexibilizing polyol, (d) and inorganic or organic monomeric or polymeric acid which acts both as reactant and catalyst, and optionally (e) a surface modifier such as an acrylic polymer containing acrylic acid.
There is continuing research effort directed to the development of improved heat-curable resin coating systems adapted for application as protective films on metal surfaces and other substrates.
Accordingly, it is an object of this invention to provide an improved water-reducible heat-curable thermosetting coating composition adapted for the protective coating of metal surfaces.
It is another object of this invention to provide a coating system which comprises a stable dispersion of heat-curable mixed resin solids in water.
It is a further object of this invention to provide a stable water-reducible epoxy resin dispersion adapted for one package baked coating applications.
Other objects and advantages of the present invention shall become apparent from the accompanying description and examples.